1. Field of the Invention
The present invention relates to image display apparatus using an electron source substrate with electron-emitting devices therein and, more particularly, to structure in a vacuum seal bonding portion.
2. Related Background Art
There are two types of conventionally known electron-emitting devices, thermal electron sources and cold cathode electron sources. The cold cathode electron sources include field emission devices (FE devices), metal/insulator/metal devices (MIM devices), surface conduction electron-emitting devices, and so on.
A surface conduction electron-emitting device will be outlined below in brief.
The foregoing surface conduction electron-emitting device, as schematically illustrated in FIGS. 17A, 17B, is comprised of a pair of device electrodes 2, 3 facing each other on a substrate 1, and an electroconductive film 4 coupled to the device electrodes and having an electron-emitting region in part thereof.
Since the above-stated surface conduction electron-emitting device is simple in structure and easy in production, it has the advantage of capability of forming an array of many devices over a large area. A variety of applications to take advantage of the feature are thus under research. For example, such applications include an electron source substrate in which a number of surface conduction electron-emitting devices are wired in a matrix pattern or the like, flat-panel image forming apparatus such as display apparatus using the electron source substrate, and so on.
FIG. 18 is a schematic illustration of a display panel constructed using the electron source substrate with a number of such electron-emitting devices therein. FIG. 18 shows the schematic sectional structure of the peripheral region of the display panel (envelope 90).
In FIG. 18, numeral 21 designates the electron source substrate with a number of electron-emitting devices (not shown) therein, which is also called a rear plate. Numeral 82 denotes a face plate in which a fluorescent film, a metal back, etc. are formed on an internal surface of a glass substrate. Numeral 86 represents a support frame.
The envelope 90 is constructed by bonding and sealing the rear plate 21, the support frame 86, and the face plate 82. The seal bonding procedure of the envelope 90 will be briefly described below.
First, the rear plate 21 and the support frame 86 are preliminarily joined to each other with frit glass 202.
Then In films 203 as a panel joining material are soldered to the support frame 86 and to the face plate 82. At this time, in order to enhance the bond strength of the In films 203 to the support frame 86 and to the face plate 82, it is desirable to provide silver paste films 204 as underlying layers.
Thereafter, the support frame 86 and the face plate 82 are joined to each other through the In films 203 at a temperature over the melting point of In in a vacuum chamber, so as to effect seal bonding, thereby forming the envelope 90.
The above-stated conventional seal bonding method of image forming apparatus, however, had the problems described below.
The joining material is In, which is a material having the relatively low melting point of 156° C. and emitting a relatively small amount of emission gas at the softening point=melting point. In use of In, there arises a problem that surface oxide films are formed in the In films 203 on the occasion of implementing ultrasonic soldering of the In films 203 to the support frame 86 and to the face plate 82, or to the silver paste films 204 as underlying layers.
Namely, the oxide films have a high melting temperature of 800° C. or more, and thus remain as oxide films even after pure In has melted during the seal bonding operation. As long as the oxide films are thin, they can break or chemically react with pure In to lose the shape of the oxide films, thus posing no problem. However, if the oxide films are thick, the uneven surface shape will remain as it is, and can give rise to vacuum leakage.
In is easy to oxidize in the atmosphere and oxygen quickly diffuses into the interior thereof at temperatures over the melting point to form a thick oxide film. Therefore, the conventional seal bonding method had the problem that the vacuum leakage was likely to occur at thick portions of the oxide films formed during the ultrasonic soldering process.
These problems can also be similarly serious problems in the case of the metals other than In or alloy materials being used as the joining material.